The aim of this project is to study the molecular organization of endogenous retroviruses. Using restriction enzyme mapping techniques, agarose gel electrophoresis, nucleic acid hybridization procedures, and transformation and transfection techniques, we have characterized the organization of endogenous ecotropic and xenotropic murine leukemia viruses in mouse chromosomal DNA. We have isolated, by preparative agar gel electrophoresis, and are cloning, using a cosmid vector system, the endogenous ecotropic proviruses present in AKR/J and BALB/c mouse DNAs. BALB/c mice contain numerous copies of endogenous xenotropic proviral DNAs which are also being molecularly cloned using cosmid vectors.